Recently the client has stopped connecting to the vpn when I login to my mac. I always have to check and manually connect. I tried removing the setup and re adding it. It then worked for a while but stopped auto connecting again yesterday.

Are there any suggestions on how I can troubleshoot this further please?

Make sure Viscosity's "Start Viscosity at Login" option is ticked (under the General section in the Preferences window). When upgrading to a major new release of macOS it's not unusual for this option to be reset (it corresponds to Viscosity being listed as a Login Item). You can confirm Viscosity is listed as a login item by going to the Apple Menu->System Preferences->Users & Groups->Select your user account to the left->Click the Login Items tab on the right.

You should then double-check that any connections you want to connect when Viscosity starts up have the "Connect when Viscosity opens" checkbox ticked.

I'm also experiencing these issues. It worked as expected until I installed Mac OS Mojave. A major problem is that it is failing silently, no error message or anything – so I just randomly detected that I wasn't connected after a while.

When investigate the logs it says "The OpenVPN subsystem could not be started."

The error message you're seeing is that OpenVPN itself is unable to launch, or Viscosity is unable to connect to it. As we're not seeing the same thing on any test machines with a fresh install of macOS 10.14, I'd be inclined to think something installed on your machine may be blocking either the OpenVPN process or Viscosity connecting to it. Perhaps slow-to-load security software, firewall software, etc. Try temporarily disabling or uninstalling any third-party software that could potentially be interfering and see if the issue persists.